Al-Anfaal (The Spoils of War)

11. (Remember) when He made the slumber fall upon you as a security from Him and sent down water from the sky upon you to purify you with it and take away from you the uncleanness of Satan and to fortify your hearts, and to steady (your) footsteps thereby.

When the Battle of Badr occurred, at first the wells of drinking water were under the control of the enemy and the Muslims were terrified. After that, it rained and the land under their feet became firm so that they would not slip, and it used them to be encouraged.

Maybe, the purpose of the Qur'anic phrase: “to steady roar footsteps” is the very ‘perseverance’, not to steady feet on the land drenched by rain.

During the war, the ability of having a slumber for a striver is a great bounty of Allah. It both removes the occasional fatigue and does not let the enemy have the opportunity of a light attack.

By the help of Allah's Will, it is also possible for a striver to have a calm-giving sleep while confronting the mass of armed enemies. But, if He does not please, there will not be a comfortable sleep possible in the best gardens either, or the sleep may not be a calm-giving one.